Recently, high precision color liquid crystal displays (LCD) have been developed for promoting office automation (OA) using multi-media computers. Such an LCD includes a 3-bit or 4-bit digital driver so as to control gradations of red (R), green (G) and blue (B) colors for each pixel. For instance, a color LCD including the 4-bit digital driver can display each of the foregoing colors in eight gradations, i.e. it can display 512 colors (=8.times.8.times.8).
Such a digital driver having the foregoing color display capacity suffices for a simple OA monitor. However, it is not sufficient to display moving and still images used on a multimedia computer, and it has been required to display images having more gradations.
In order to meet this requirement, a variety of proposals have been made to diffuse image components, which cannot be displayed by one pixel, to adjacent pixels in the same image frame (i.e. inframe error diffusion) and to increase the number of gradations using a quantized continuous tone image processing technique.
In this specification, the term "error data" denotes data, out of image data, which are represented by lower bits and are not provided to a digital driver and are not displayed on an LCD. For example, image data are represented by six bits while the digital driver is four-bit, so the lower two bits are error data.
FIG. 12 of the accompanying drawings shows an example of the error diffusing apparatuses of the prior art. This error diffusing apparatus employs the inframe error diffusion technique, and processes data of one color image. This apparatus stores error data which are the lower bits of image data in one pixel and are not displayed on an LCD, and adds the error data to image data of a succeeding pixel. The apparatus performs the quantized continuous tone image processing using the error diffusing method.
Referring to FIG. 12, a latch circuit 11 latches 6-bit original image data GD arriving in synchronization with dot clocks DCK, and provides the image data GD to an arithmetic circuit 12. The arithmetic circuit 12 adds the image data GD and error data E1 from an error data storage circuit 13, creating corrected 6-bit image data. The error data storage circuit 13 stores the two lower bits of the corrected 6-bit image data in response to the dot clock DCK, and outputs the two lower bits to the arithmetic circuit 12 when original image data GD of a next pixel are latched in the latch circuit 11. Four higher bits of the corrected 6-bit image data are outputted as image display data HD to an output latch circuit 14. The image is displayed on the basis of the 4-bit image display data HD, and the two remaining lower bit error data are sequentially diffused into adjacent pixels. Thus, a halftone image is displayed since brightness levels of a plurality of pixels are averaged.
For example, assume that the original image data have a value of "100010" for all pixels. First of all, "00" of error data EI is added to the original image data "100010", so corrected 6-bit image data "100010" will be created. The two lower bits "10" of the corrected 6-bit image data are stored as error data EI in the error data storage circuit 13. The four higher bits "1000" are output as image display data HD. Then, the error data EI "10" is added to next original image data GD "100010", so corrected image data will be "100100". The error data EI "00" is stored, and image display data HD "1001" is outputted. The foregoing operation is repeated, so "1000" and "1001" are alternately displayed by each pixel. This means that two pixels display a halftone image. A quarter-tone image represented by the least significant bit (LSB) of the original image data GD is displayed by four pixels of corrected image data (i.e. one of four pixels of corrected image data has the least significant bit LSB which is 1).
The foregoing error diffusion is applied to the colors R, G and B, respectively, so each of them can be displayed in 64 (6 bits) gradations similarly to the original image data GD. This error diffusion technique is effective in remarkably improving the quality of a natural image which is moving or has varying color densities.
However, since the error data are laterally added in the foregoing error diffusion technique, data of a left image tends to adversely affect those of a right image, so image display data will be affected accordingly. When a displayed image is flat, error data which are caused by discontinuous variations of left image data affect the right image in a discernible manner. The left image tends to have a poor quality. For instance, when a cursor moves on a flat color background of a personal computer display, it seems as if it has a tail. In such a case, an error in the image data of the cursor appears at the right of the display screen far from the cursor itself, and an artifact appears there.
In order to overcome the foregoing problem, in the prior art, not only error data which are stored for every given pixels are periodically reset but also a border (i.e. an edge) of the image is detected. When the edge is detected, the existing error data are reset. Thus, the image is protected against being affected by irrelevant error data when it is displayed. Further, a difference between image data of a preceding pixel and image data currently supplied is calculated. If the difference is more than a given value, an image edge is identified.
However, if the error data are reset when an image edge is detected, no error data will be stored for an image after the detection of the edge, so the error diffusion is not effective for an image present around the edge. This tends to worsen the quality of the image. Especially, a computer-created image is sometimes displayed by superimposing flat images having different levels of tone or brightness. In such a case, an image around the edge (pixels succeeding the edge pixel) often suffers from poor quality. Further, if a line or the like having a slightly different brightness (e.g. the difference is represented by the number of bits in error data) is present in a flat image, a border of such a line is detected as an edge, on the basis of which error data may be reset. Thus, no carry signal is generated in the corrected image data on the basis of the resetting of the error data, so the line may not be displayed.
In the case of a computer-created image, different image data are alternately applied to every pixel so as to indicate an image having a tone or brightness which cannot be indicated by a display. Such an image is displayed by interpolated tone or brightness, and is called a "checkered pattern". Further, there is a checkered pattern formed by using two pixels. Since error data are periodically reset in the checkered pattern, the carry signals are generated at certain pixels by addition of the error data. This carry causes a particular pattern.
Therefore, it is preferable to change the contents of the error diffusion in accordance with the contents of the computer-created image.